The present invention relates to a planetary gearbox, in particular for a wind turbine, with a gearbox housing, a central sunwheel which is mounted in the gearbox housing so that it can rotate about a central gearbox axis and has teeth on its outer side, a ringwheel which is arranged concentrically to the central gearbox axis within the gearbox housing and has teeth on its inner side, a planet gear carrier which is mounted in the gearbox housing so that it can rotate about the central gearbox axis, and several planet gears which are mounted by planet gear bearings on the planet gear carrier so that they can turn about planetary gear pins and which have teeth on their outer sides, these being meshed with the inner teeth of the ring gear and the outer teeth of the sunwheel.
Externally, there is hardly any difference between modern wind turbines. The differences lie essentially in the arrangement of the drives, which are provided between the rotor and the generator for the purpose of converting the low rotational speed of the rotor hub into a higher rotational speed for the generator. For large turbines, conversion ratios of 1:100 are usual. In order to realize such large conversion ratios, use is generally made of multi-stage gearboxes. Here, the preference in the current drive concepts is for a combination of planetary gear stages and spur gear stages, wherein in the first, high-torque, gearbox stages use is often made of obliquely toothed planetary gears for a compact construction, while in the subsequent high-power stages use is made of an obliquely toothed spur gear gearbox.
Here, the rotor shaft or rotor hub, as applicable, is joined to the planetary gear carrier of the first stage. This has bearing mounts in the gearbox housing at two positions on axially opposite sides of the planetary gears—i.e. on the drive input side and on the drive offtake side of the gearbox stage—and drives the planetary gears. Alternatively, a one-side bearing mount is also possible. The ringwheel/ring gear has a fixed joint to the gearbox housing, so that the result is a circulatory movement of the planet gears in the gearbox. The torque is transmitted from the planet gears to the sunwheel/sun gear, which is joined to the planet gear carrier of the second stage by a splined shaft. The action of the second planetary stage is identical to the first stage. In the last gearbox stage the spur gear, which is driven by the sun gear of the second stage, effects a further conversion, so that a conversion takes place from an initially small rate of rotation of the rotor shaft with a high torque to a high rate of rotation of the generator shaft with a low torque.
In the case of conventional planetary gearboxes, and planet-spur-gear gearboxes for wind turbines, use is mainly made of roller bearings for the bearing mounts of the planet gears in the planet gear carrier and also the planet gear carrier in the gearbox housing.
Occasionally, use is also made of hydrodynamically operating sliding bearings. EP 2 302 257 A2 discloses, for example, a planetary gearbox of the type cited in the introduction with a gearbox housing and a central sunwheel, which is mounted in the gearbox housing so that it can rotate about a central gearbox axis, and has external teeth. Further, a ringwheel is provided which is arranged in the gearbox housing concentrically with the central gearbox axis, and which has internal teeth. Also provided is a planet gear carrier, which has a bearing mounting in the gearbox housing so that it can rotate about the central gearbox axis. On the planet gear carrier are several planet gears, which have planet gear bearing mountings so that they can rotate about planetary gear wirier pins. The planet gears have external teeth, which mesh with the internal teeth on the ringwheel and the external teeth on the sunwheel. In this planetary gearbox, the planet gear carrier is supported on the gearbox housing by radial sliding bearings.
Known materials for sliding bearings, which are used for sliding bearings, are for example bearing metal with alloying components and bronze alloys. In general, sliding bearings for industrial applications are arranged with a lubrication groove of about 15 to 20 μm relative to the diameter at the operating point. For bearing metal, the permissible mean dynamic pressure specified by bearing manufacturers is at least 5 MPa.
However, the use of sliding bearings is relatively rare. The reasons for this are the unstable operating conditions which predominate in many cases, and temporary exceptionally low sliding speeds at the same time as extreme loading on the sliding bearing. Conventional sliding bearings are mainly used when the application conditions involve high to very high rotational speeds. For these reasons it is usual to use almost exclusively roller bearings for bearing sites in wind turbines.
Regardless of whether roller bearings or sliding bearings are used, the attempt is made to avoid mixed friction in operation by lubrication measures, and in this way to keep the wear at the bearing sites as low as possible. However, the measures which must be taken for lubrication and cooling of the drive components, in particular in the region of the bearing sites, are very costly. Apart from this, in spite of all countermeasures all the bearing sites are subject to slight wear, because of the high loadings and the low rotational speeds involved about its own axle for the rotor of a wind turbine, because they regularly operate in the region of mixed friction. The abrasion from the gears, which is present and has not yet been filtered out, accelerates yet more the wear at the contact surfaces and thus limits the service life of the bearing sites.